Solar cells are typically fabricated as separate physical entities with light gathering surface areas on the order of 4-6 cm2 or larger. For this reason, it is standard practice for power generating applications to mount the cells in a flat array on a supporting substrate or panel so that their light gathering surfaces provide an approximation of a single large light gathering surface. Also, since each cell itself generates only a small amount of power, the required voltage and/or current is realized by interconnecting the cells of the array in a series and/or parallel matrix.
A conventional prior art solar cell structure is shown in FIG. 1. Because of the large range in the thickness of the different layers, they are depicted schematically. Moreover, FIG. 1 is highly schematic so that it represents the features of both “thick-film” solar cells and “thin-film” solar cells. In general, solar cells that use an indirect band gap material to absorb light are typically configured as “thick-film” solar cells because a thick film of the absorber layer is required to absorb a sufficient amount of light. Solar cells that use a direct band gap material to absorb light are typically configured as “thin-film” solar cells because only a thin layer of the direct band-gap material is needed to absorb a sufficient amount of light.
The arrows at the top of FIG. 1 show the source of direct solar illumination on the cell. Layer 102 is the substrate. Glass or metal is a common substrate. In thin-film solar cells, substrate 102 can be-a polymer-based backing, metal, or glass. In some instances, there is an encapsulation layer (not shown) coating substrate 102. Layer 104 is the back electrical contact for the solar cell.
Layer 106 is the semiconductor absorber layer. Back electrical contact 104 makes ohmic contact with absorber layer 106. In many but not all cases, absorber layer 106 is a p-type semiconductor. Absorber layer 106 is thick enough to absorb light. Layer 108 is the semiconductor junction partner-that, together with semiconductor absorber layer 106, completes the formation of a p-n junction. A p-n junction is a common type of junction found in solar cells. In p-n junction based solar cells, when semiconductor absorber layer 106 is a p-type doped material, junction partner 108 is an n-type doped material. Conversely, when semiconductor absorber layer 106 is an n-type doped material, junction partner 108 is a p-type doped material. Generally, junction partner 108 is much thinner than absorber layer 106. For example, in some instances junction partner 108 has a thickness of about 0.05 microns. Junction partner 108 is highly transparent to solar radiation. Junction partner 108 is also known as the window layer, since it lets the light pass down to absorber layer 106.
In a typical thick-film solar cell, absorber layer 106 and window layer 108 can be made from the same semiconductor material but have different carrier types (dopants) and/or carrier concentrations in order to give the two layers their distinct p-type and n-type properties. In thin-film solar cells in which copper-indium-gallium-diselenide (CIGS) is the absorber layer 106, the use of CdS to form junction partner 108 has resulted in high efficiency cells. Other materials that can be used for junction partner 108 include, but are not limited to, In2Se3, In2S3, ZnS, ZnSe, CdInS, CdZnS, ZnIn2Se4, Zn1-xMgxO, CdS, SnO2, ZnO, ZrO2 and doped ZnO.
Layer 110 is the counter electrode, which completes the functioning cell. Counter electrode 110 is used to draw current away from the junction since junction partner 108 is generally too resistive to serve this function. As such, counter electrode 110 should be highly conductive and transparent to light. Counter electrode 110 can in fact be a comb-like structure of metal printed onto layer 108 rather than forming a discrete layer. Counter electrode 110 is typically a transparent conductive oxide (TCO) such as doped zinc oxide (e.g., aluminum doped zinc oxide, gallium doped zinc oxide, boron doped zinc oxide), indium-tin-oxide (ITO), tin oxide (SnO2), or indium-zinc oxide. However, even when a TCO layer is present, a bus bar network 114 is typically needed in conventional solar cells to draw off current since the TCO has too much resistance to efficiently perform this function in larger solar cells. Network 114 shortens the distance charge carriers must move in the TCO layer in order to reach the metal contact, thereby reducing resistive losses. The metal bus bars, also termed grid lines, can be made of any reasonably conductive metal such as, for example, silver, steel or aluminum. In the design of network 114, there is design a trade off between thicker grid lines that are more electrically conductive but block more light, and thin grid lines that are less electrically conductive but block less, light. The metal bars are preferably configured in a comb-like arrangement to permit light rays through TCO layer 110. Bus bar network layer 114 and TCO layer 110, combined, act as a single metallurgical unit, functionally interfacing with a first ohmic contact to form a current collection circuit. In U.S. Pat. No. 6,548,751 to Sverdrup et al., hereby incorporated by reference herein in its entirety, a combined silver bus bar network and indium-tin-oxide layer function as a single, transparent ITO/Ag layer.
Layer 112 is an antireflective coating that can allow a significant amount of extra light into the cell. Depending on the intended use of the cell, it might be deposited directly on the top conductor as illustrated in FIG. 1. Alternatively or additionally, antireflective coating 112 made be deposited on a separate cover glass that overlays top electrode 110. Ideally, the antireflective coating reduces the reflection of the cell to very near zero over the spectral region in which photoelectric absorption occurs, and at the same time increases the reflection in the other spectral regions to reduce heating. U.S. Pat. No. 6,107,564 to Aguilera et al., hereby incorporated by reference herein in its entirety, describes representative antireflective coatings that are known in the art.
Solar cells typically produce only a small voltage. For example, silicon based solar cells produce a voltage of about 0.6 volts (V). Thus, solar cells are interconnected in series or parallel in order to achieve greater voltages. When connected in series, voltages of individual cells add together while current remains the same. Thus, solar cells arranged in series reduce the amount of current flow through such cells, compared to analogous solar cells arrange in parallel, thereby improving efficiency. As illustrated in FIG. 1, the arrangement of solar cells in series is accomplished using interconnects 116. In general, an interconnect 116 places the first electrode of one solar cell in electrical communication with the counter-electrode of an adjoining solar cell.
As noted above and as illustrated in FIG. 1, conventional solar cells are typically in the form of a plate structure. Although such cells are highly efficient when they are smaller, larger planar solar cells have reduced efficiency because it is harder to make the semiconductor films that form the junction in such solar cells uniform. Furthermore, the occurrence of pinholes and similar flaws increase in larger planar solar cells. These features can cause shunts across the junction.
A number of problems are associated with solar cell designs present in the known art. A number of prior art solar cell designs and some of the disadvantages of each design will now be discussed.
As illustrated in FIG. 2A, U.S. Pat. No. 6,762,359 B2 to Asia et al. discloses a solar cell 210 including a p-type layer 12 and an n-type layer 14. A first electrode 32 is provided on one side of the solar cell. Electrode 32 is in electrical contact with n-type layer 14 of solar cell 210. Second electrode 60 is on the opposing side of the solar cell. Electrode 60 is in electrical contact with the p-type layer of the solar cell. Light-transmitting layers 200 and 202 form one side of device 210 while layer 62 forms the other side. Electrodes 32 and 60 are separated by insulators 40 and 50. In some instances, the solar cell has a tubular shape rather than the spherical shape illustrated in FIG. 2. While device 210 is functional, it is unsatisfactory. Electrode 60 has to pierce absorber 12 in order to make an electrical contact. This results in a net loss in absorber area, making the solar cell less efficient. Furthermore, such a junction is difficult to make relative to other solar cell designs.
As illustrated in FIG. 2B, U.S. Pat. No. 3,976,508 to Mlaysky discloses a tubular solar cell comprising a cylindrical silicon tube 2 of n-type conductivity that has been subjected to diffusion of boron into its outer surface to form an outer p-conductivity type region 4 and thus a p-n junction 6. The inner surface of the cylindrical tube is provided with a first electrode in the form of an adherent metal conductive film 8 that forms an ohmic contact with the tube. Film 8 covers the entire inner surface of the tube and consists of a selected metal or metal alloy having relatively high conductivity, e.g., gold, nickel, aluminum, copper or the like, as disclosed in U.S. Pat. Nos. 2,984,775, 3,046,324 and 3,005,862. The outer surface is provided with a second electrode in the form of a grid consisting of a plurality of circumferentially extending conductors 10 that are connected together by one or more longitudinally-extending conductors 12. The opposite ends of the outer surface of the hollow tube are provided with two circumferentially-extending terminal conductors 14 and 16 that intercept the longitudinally-extending conductors 12. The spacing of the circumferentially-extending conductors 10 and the longitudinally-extending conductors 12 is such as to leave areas 18 of the outer surface of the tube exposed to solar radiation. Conductors 12, 14 and 16 are made wider than the circumferentially-extending conductors 10 since they carry a greater current than any of the latter. These conductors are made of an adherent metal film like the inner electrode 8 and form ohmic contacts with the outer surface of the tube. While the solar cell disclosed in FIG. 2 is functional, it is also unsatisfactory. Conductors 12, 14, and 16 are not transparent to light and therefore the amount of light that the solar cell receives is reduced.
U.S. Pat. No. 3,990,914 to Weinstein and Lee discloses another form of tubular solar cell. Like Mlaysky, the Weinsten and Lee solar cell has a hollow core. However, unlike Mlaysky, Weinstein and Lee dispose the solar cell on a glass tubular support member. The Weinstein and Lee solar cell has the drawback of being bulky and expensive to build.
Referring to FIGS. 2C and 2D, Japanese Patent Application Kokai Publication Number S59-125670, Toppan Printing Company, published Jul. 20, 1984 (hereinafter “S59-125670”) discloses a rod-shaped solar cell. The rod shaped solar cell is depicted in cross-section in FIG. 2C. A conducting metal is used as core 1 of the cell. A light-activated amorphous silicon semiconductor layer 3 is provided on core 1. An electrically conductive transparent conductive layer 4 is built up on top of semiconductor layer 3. The transparent conductive layer 4 can be made of materials such as indium oxide, tin oxide or indium tin oxide (ITO) and the like. As illustrated in FIG. 2C, a layer 5, made of a good electrical conductor, is provided on the lower portion of the solar cell. The publication states that this good conductive layer 5 is not particularly necessary but helps to lower the contact resistance between the rod and a conductive substrate 7 that serves as a counter-electrode. As such, conductive layer 5 serves as a current collector that supplements the conductivity of counter-electrode 7 illustrated in FIG. 2D.
As illustrated in FIG. 2D, rod-shaped solar cells 6 are multiply arranged in a row parallel with each other, and counter-electrode layer 7 is provided on the surface of the rods that is not irradiated by light so as to electrically make contact with each transparent conductive layer 4. The rod-shaped solar cells 6 are arranged in parallel and both ends of the solar cells are hardened with resin or a similar material in order to fix the rods in place.
S59-125670 addresses many of the drawbacks associated with planar solar cells. However, S59-125670 has a number of significant drawbacks that limit the efficiency of the disclosed devices. First, the manner in which current is drawn off the exterior surface is inefficient because layer 5 does not wrap all the way around the rod (e.g., see FIG. 2C). Second, substrate 7 is a metal plate that does not permit the passage of light. Thus, a full side of each rod is not exposed to light and can thus serve as a leakage path. Such a leakage path reduces the efficiency of the solar cell. For example, any such dark junction areas will result in a leakage that will detract from the photocurrent of the cell. Another disadvantage with the design disclosed in FIGS. 2C and 2D is that the rods are arranged in parallel rather than in series. Thus, the current levels in such devices will be large, relative to a corresponding serially arranged model, and therefore subject to resistive losses.
Referring to FIG. 2E, German Unexamined Patent Application DE 43 39 547 A1 to Twin Solar-Technik Entwicklungs-GmbH, published May 24, 1995, (hereinafter “Twin Solar”) also discloses a plurality of rod-shaped solar cells 2 arranged in a parallel manner inside a transparent sheet 28, which forms the body of the solar cell. Thus, Twin Solar does not have some of the drawbacks found in S59-125670. Transparent sheet 28 allows light in from both faces 47A and 47B. Transparent sheet 28 is installed at a distance from a wall 27 in such a manner as to provide an air gap 26 through which liquid coolant can flow. Thus, Twin Solar devices have the drawback that they are not truly bifacial. In other words, only face 47A of the Twin Solar device is capable of receiving direct light. As defined here, “direct light” is light that has not passed through any media other than air. For example, light that has passed through a transparent substrate, into a solar cell assembly and exited the assembly, is no longer direct light once it exits the solar cell assembly. Light that has merely reflected off of a surface, however, is direct light provided that it has not passed through a solar cell assembly. Under this definition of direct light, face 47B is not configured to receive direct light. This is because all light received by face 47B must first traverse the body of the solar cell apparatus after entering the solar cell apparatus through face 47A. Such light must then traverse cooling chamber 26, reflect off back wall 42, and finally re-enter the solar cell through face 47B. The solar cell assembly is therefore inefficient because direct light cannot enter both sides of the assembly.
Although tubular designs of solar cells have addressed many of the drawbacks associated with planar solar cells, some problems remain unresolved. The capacity of solar cells to withstand physical shock is one unresolved problem. Conventional solar cell panels often crack after a certain number of years, often even before the gained energy benefit can balance their production costs. Solar cell assemblies are often built from small individual solar cell units. This approach provides efficiency and flexibility. Smaller solar cells are easier to manufacture at a large scale, and they can also be assembled into different sizes and shapes to suit the ultimate application. Inevitably, the smaller solar cell unit design also comes with the price of fragility. The smaller solar cell units easily break under pressure during transportation or routine handling processes. What are needed in the art are methods and systems that provide support and strength to solar cell units while maintaining the advantages of the small design.
Discussion or citation of a reference herein will not be construed as an admission that such reference is prior art to the present invention.